Keep Markdown preview tabs open across buffers

[CollieIsCute]

Summary

• disable markdown-preview.nvim auto-close when switching buffers
• keep combine preview disabled so each Markdown file can use its own browser tab

Verification

• luac -p home/dot_config/nvim/lua/plugins/markdown-preview.lua
• XDG_CONFIG_HOME=/home/collie/.local/share/chezmoi/home/dot_config nvim --headless "+lua assert(vim.g.mkdp_auto_close == 0, 'mkdp_auto_close not disabled'); assert(vim.g.mkdp_combine_preview == 0, 'mkdp_combine_preview not disabled')" "+qa"